1. Field of the Invention
The present invention relates to a structure having pores, particularly to a structure useful for forming a recording layer of magnetic recording media.
The present invention relates also to a process for producing a structure, in particular magnetic recording media, in which magnetic ordered alloy particles are arranged in a non-magnetic material.
2. Description of Related Art
With remarkable increase of the amount of information in recent years, magnetic recording devices like hard disk drives (HDDs) are required to have a higher recording density and a larger recording capacity. In a recording apparatus, constitution of the recording unit (one bit) from many magnetic particles improves the recording resolution and decreases the recording noise to realize a higher recording density. For formation of such a recording unit, the size of the magnetic particles in the recording medium should be micronized. However, the micronization of the magnetic fine particles makes significant the thermal energy retained in the particle relative to the magnetic energy. This produces a superparamagnetic effect (thermal fluctuation) to cause dissipation of magnetic record disadvantageously. To stabilize the magnetic record against the thermal fluctuation, magnetic recording media are being developed which are constituted of fine particles having a large magnetic anisotropy constant (Ku).
The hard disk which is a main recording device of a personal computer has been improved to have remarkably high recording density. The hard disks are investigated for use as the recording medium, not only for the PC but also for digital household electric appliances and mobile terminals, and are promising for higher recording density.
The hard disk conventionally used is of a longitudinal recording system in which magnetization is held in the disk face direction in the disk. In this system, the magnetic recording layer should be thin to prevent decrease of the demagnetization field in the magnetic domains. The decrease of the thickness of the magnetic recording layer results in decrease of the volume of the magnetic particles contained therein. This makes non-negligible the thermal energy of the particles in comparison with the magnetic energy retained therein. That is, the longitudinal recording type of hard disk having a thin magnetic recording medium will be affected remarkably by superparamagnetism (thermal fluctuation) to dissipate the recorded magnetization.
On the contrary, in the perpendicular recording system in which magnetization is held in a direction vertical to the disk face, the superparamagnetization can be suppressed by keeping the thickness of the layer.
The recording layer of the perpendicular magnetic recording media is conventionally formed mainly from a CoCr type alloy. Recently, however, hard magnetic ordered alloys of a CuAu type (hereinafter referred to as an “L10 type”) or a Cu3Au type (hereinafter referred to as an “L12 type”) are attracting attention which are capable of suppressing the superparamagnetization even with a smaller size of the recording region and have a high magnetic anisotropic constant.
The material FePt can become an L10 type ordered alloy. For formation of the L10 type FePt ordered alloy, a film thereof is heat-treated for ordering. Japanese Patent Application Laid-Open No. 2003-006830 (Patent Document 1) discloses a process in which a continuous film composed of Fe and Pt is formed on a substrate and the film is heat treated at 350° C.
For increasing the recording density, the magnetic exchange bond between the magnetic regions should be broken or weakened. For the purpose, it is effective to isolate the magnetic regions from each other by a non-magnetic material composed of an oxide or the like. Japanese Patent Application Laid-Open No. 2002-175621 (Patent Document 2) prepares an ordered alloy structure by filling a magnetic material like CoPt into pores of a structure constituted of anodized alumina and heat-treating the structure at a temperature of 650° C. This heat treatment temperature is higher than the temperature 350° C. for the ordering of the continuous film as described in Patent Document 1. Therefore, improvement is desired at least to lower the heat treatment temperature below 650° C.
(0004) L10-ordered alloys such as FePt and CoPt having an L10 structure are noticed as the material having a high magnetic anisotropy constant of not lower than 1×107 erg/cm3. In particular, the L10-FePt alloy is noticed which has a magnetic anisotropy constant as high as 7×107 erg/cm3. A film of the FePt alloy formed at room temperature has an fcc disordered crystal structure. This structure will be transformed by heat treatment into an fct ordered structure (L10 structure). However, the heat treatment can increase the crystal particle size.
The term “a granular structure” signifies a dispersion of spherical crystal members 1022 in matrix member 1012 as illustrated in FIG. 8. Granular structures are being investigated for preparing an ordered alloy structure with simultaneous inhibition of crystal grain growth. The term “a nano-granular structure” signifies a dispersion of nanometric crystal grains in a matrix such as oxides (SiO2, Al2O3, MgO, etc.).
Japanese Patent Application Laid-Open No. 2001-273622 discloses magnetic recording media employing a nano-granular structure. However, in production of this recording media, crystal particles will grow with progress of ordering of the alloy, making difficult to obtain the granular structure containing L10-FePt crystal particles of the size of not larger than 10 nm.
Japanese Patent No. 3507892 discloses formation of an alloy of Fe and Pt by lamination of a granular thin film containing Fe fine particles and another granular thin film containing Pt fine particles, and heating up to a prescribed temperature during or after the lamination. This patent document describes a granular structure containing L10-FePt crystal grains of 10 nm having coersive force (Hc) of not less than 5 kOe obtained by heating to 350° C. or higher. This method, however, still requires heating up to 350° C. or higher for the ordering of the alloy For practical application, ordering of the alloy of the magnetic material is required to be achieved at a lower temperature. Some of the techniques disclosed until now for lowering the temperature for ordering a FePt alloy in a continuous film are described below.
(1) Applied Physics Letters (Mar. 25, 2002), vol. 80, No. 12, pp. 2147-2149, and Journal of Applied Physics (Nov. 15, 2002), vol. 92, No. 10, pp. 6104-6109 disclose addition of a third element such as Cu, Ag, Au, Ir, B, and N to an FePt alloy. Of the third element, Cu and the like become effective by formation of a solid solution in FePt: Ag, Au, and the like are effective in formation of empty holes in the particle by destroying the solid solution.
(2) Applied Physics Letters (Jan. 14, 2002), vol. 80, No. 2, pp. 288-290 discloses alternate lamination of Fe monoatomic layers and Pt monoatomic layers for forming an ordered alloy layer. L10-FePt has a structure of lamination of Fe and Pt in layers. Controlled lamination in an atomic order enables decrease of the energy for transformation into an ordered structure.
(3) Journal of Applied Physics (Jun. 1, 2001), vol. 89, No. 11, pp. 7065-7067, and Journal of Applied Physics (Dec. 1, 2003), vol. 94, No. 11, pp. 7222-7226 disclose a method of lamination of Fe and Pt in films of several nanometers thick. This method also enables lowering of the ordering temperature by a reason similar to that of the above Method-(2), particularly effectively in film formation with heating.
(4) Applied Physics Letters (Nov. 8, 2004), vol. 85. No. 19, pp. 4430-4432 discloses a method for promotion of the ordering by utilizing the strain energy produced in formation of Cu3Si from Cu and Si. In this method, a Cu film is formed on a Si substrate at room temperature, and is heat-treated: The dynamic tensile force produced in silicide formation by the heating is effective for the ordering. This method, when applied in the present invention, a similar effect can be achieved by formation of a Cu/Si lamination film on a continuous FePt film.
(5) Applied Physics Letters (Dec. 1, 2003), vol. 83, No. 22, pp. 4550-4552 discloses irradiation of He ions in place of the heat treatment for lowering the ordering temperature.
(6) Applied Physics Letters (Sep. 20, 2004), vol. 85, No. 12, pp. 2304-2306 discloses heat treatment in a magnetic field for promoting the ordering.
(7) Journal of Applied Physics (Jun. 1, 2001), vol. 89, No. 11, pp. 7068-7070, and Applied Physics Letters (Sep. 15, 2003, vol. 83, No. 11, pp. 2196-2198 disclose use of a Ag thin films as the underlying layer and a cap layer for lowering the ordering temperature. Presumably this lowering of the ordering temperature is caused by an elastic energy produced in the interface between FePt and the Ag film as one.
(8) Other methods include increase of the Ar pressure in FePt film formation to promote diffusion of Fe and Pt; increase of the vacuum degree in the film forming apparatus to decrease an impurity not to inhibit the progress of ordering; and decrease of an impurity concentration in the target employed. By any of the above-mentioned methods, the ordering temperature can be lowered by optimizing basic conditions of the film formation.